The thermal recording device which forms images with a dot matrix by using a thermal head is conventionally known, and such a thermal recording device forms images by applying a thermal head consisting of a plurality of heat emitting elements onto thermal recording paper, an OHP coloring TP sheet, an OHP frosted TP sheet, recording paper in conjunction with the use of thermal transfer ribbon, or a recording surface of heat sensitive recording material such as a thermal stencil master plate, and by selectively heating the heat emitting elements. Such thermal recording devices are widely used as facsimiles, printers for ticket dispensers, hand-held copiers, OHP transparency making devices, and thermal master plate making devices.
In facsimiles, the feed speed of the thermal recording paper in the longitudinal direction or in the secondary scanning direction is determined by a unified standard, and the size of each heat emitting element is determined according to the feed speed in the secondary scanning direction. Further, the aspect ratio of each heat emitting element is determined to be a/b=1/2 by a communication standard where a and b are the lengths of each heat emitting element in the primary and secondary scanning directions, respectively, the primary direction corresponding to the lateral direction of the paper or the direction of the row of the heat emitting elements.
Therefore, in the high resolution mode (fine mode) of the facsimile standard in which Pa=Pb where the dot pitch in the primary scanning direction is Pa and the dot pitch in the secondary scanning direction is Pb, b&gt;Pa=Pb holds, and there will be some overlapping in the heat emitting regions of the heat emitting elements for each small distance along the secondary scanning direction.
If a thermal stencil master plate is processed or made by forming stencil images on a thermo-plastic film of a thermal stencil master plate with a thermal head for a facsimile of the above described kind in a mode equivalent to the high resolution mode of the facsimile standard, continuous openings will be formed in the thermo-plastic film of the thermal stencil master plate along the secondary scanning direction due to the above-mentioned overlapping. This causes not only the thickening and blurring of the lines of printed character and line images but also excessive deposition of ink onto the printing paper in solid areas of the picture images which could in turn cause conspicuous smearing of the reverse surface of the printing paper by ink transfer in continuous printing.
To overcome this problem, it has been proposed to make a thermal stencil master plate with a thermal head using heat emitting elements each of which is shorter in length along the secondary scanning direction than the pitch of the secondary scanning, and to ensure formation of unaffected parts between the perforations along the secondary scanning direction as disclosed in Japanese patent laid open publication No. 2-67133.
According to this proposal, since the perforations formed in the thermoplastic resin film of the thermal stencil master plate are formed so as to be independent from each other in both primary and secondary scanning directions, it is possible to faithfully reproduce character images by printing, and to control excessive deposition of ink and reduce ink transfer from one sheet to another.
However, images formed by perforation of a film of a thermal stencil master plate are inferior in quality as compared to those formed by using thermal coloring type media, such as thermal recording paper, in terms of reproducibility (resolution) as compared to the original images, in particular the evenness of fine lines and small characters, legibility of small outlined characters, the sharpness of fine black and white patterns such as halftone screen images, and digitally reproduced photographic gradations.
Further, in a high temperature environment, the perforation of the thermo-plastic resin film of the thermal stencil master plate, due to melting, tends to be excessive, and, combined with the lowering of the viscosity of the ink, the thickening and blurring of lines of character images become more pronounced, as compared to the original images, than in a normal or low temperature environment. Additionally, the smearing or the ink transfer of the printing paper tends to be more pronounced due to increase in the amount of ink deposition, and the acceptable temperature range becomes narrower.
Thus, it has not heretofore been possible to provide a thermal recording device which can achieve the picture quality equivalent or comparable to those of the picture images produced by the coloring of thermal recording paper in the picture images produced by using the thermal stencil master plate, and achieving a desired uniformity in picture quality even when thermal recording materials having different recording properties are used.